A superficially similar but entirely unrelated flowering plant genus, mare's tail (Hippuris), is occasionally referred to as "horsetail", and adding to confusion, the name mare's tail is sometimes applied to Equisetum.[4]

It has been suggested that the pattern of spacing of nodes in horsetails, wherein those toward the apex of the shoot are increasingly close together, inspired John Napier to discover logarithms.[5]

The name "horsetail", often used for the entire group, arose because the branched species somewhat resemble a horse's tail. Similarly, the scientific nameEquisetum derives from the Latinequus ("horse") + seta ("bristle").

Other names include candock for branching individuals, and snake grass or scouring-rush for unbranched or sparsely branched individuals. The latter name refers to the rush-like appearance of the plants, and to the fact that the stems are coated with abrasive silicates, making them useful for scouring (cleaning) metal items such as cooking pots or drinking mugs, particularly those made of tin. In German, the corresponding name is Zinnkraut ("tin-herb"). Rough horsetail E. hyemale is still boiled and then dried in Japan, to be used for the final polishing process on woodcraft to produce a smoother finish than any sandpaper.

In these plants the leaves are greatly reduced and usually non-photosynthetic. They contain a single, non-branching vascular trace, which is the defining feature of microphylls. However, it has recently been recognised that horsetail microphylls are probably not ancestral as in Lycopodiophyta (clubmosses and relatives), but rather derived adaptations, evolved by reduction of megaphylls.[6] They are, therefore, sometimes referred to as megaphylls to reflect this homology.

The leaves of horsetails are arranged in whorls fused into nodal sheaths. The stems are green and photosynthetic, and are distinctive in being hollow, jointed and ridged (with sometimes 3 but usually 6-40 ridges). There may or may not be whorls of branches at the nodes.

The spores are borne under sporangiophores in strobili, cone-like structures at the tips of some of the stems. In many species the cone-bearing shoots are unbranched, and in some (e.g. field horsetail, E. arvense) they are non-photosynthetic, produced early in spring separately from photosynthetic, sterile shoots. In some other species (e.g. marsh horsetail, E. palustre) they are very similar to sterile shoots, photosynthetic and with whorls of branches.

Horsetails are mostly homosporous, though in the field horsetail smaller spores give rise to male prothalli. The spores have four elaters that act as moisture-sensitive springs, assisting spore dispersal after the sporangia have split open longitudinally.

The crude cell extracts of all Equisetum species tested contain mixed-linkage glucan : Xyloglucan endotransglucosylase (MXE) activity.[7] This is a novel enzyme and is not known to occur in any other plants. In addition, the cell walls of all Equisetum species tested contain mixed-linkage glucan (MLG), a polysaccharide which, until recently, was thought to be confined to the Poales.[8][9] The evolutionary distance between Equisetum and the Poales suggests that each evolved MLG independently. The presence of MXE activity in Equisetum suggests that they have evolved MLG along with some mechanism of cell wall modification. The lack of MXE in the Poales suggests that there it must play some other, currently unknown, role. Due to the correlation between MXE activity and cell age, MXE has been proposed to promote the cessation of cell expansion.

The living members of the genus Equisetum are divided into two distinct lineages, which are usually treated as subgenera. Hybrids are common, but hybridization has only been recorded between members of the same subgenus.[10]

Many species in this genus prefer wet sandysoils, though some are semi-aquatic and others are adapted to wet clay soils. The stalks arise from rhizomes that are deep underground and almost impossible to dig out. The field horsetail (E. arvense) can be a nuisance weed, readily regrowing from the rhizome after being pulled out. It is also unaffected by many herbicides designed to kill seed plants. However, as E. arvense prefers an acid soil, lime may be used to assist in eradication efforts to bring the soil pH to 7 or 8.[12] Members of the genus have been declared noxious weeds in Australia and in the US state of Oregon.[13][14]

If eaten over a long enough period of time, some species of horsetail can be poisonous to grazing animals, including horses.[15] The toxicity appears to be due to thiaminase enzymes, which can cause thiamine (vitamin B1) deficiency.[16][17][18][19] People have regularly consumed horsetails. The young plants are eaten cooked or raw, but considerable care must be taken.[20] For example, the fertile stems bearing strobili of some species are cooked and eaten like asparagus (a dish called tsukushi[21]) in Japan.[22] The people of ancient Rome would eat meadow horsetail in a similar manner, and they also used it to make tea as well as a thickening powder.[23] Native Americans in the Pacific Northwest eat the young shoots of this plant raw.[24] The plants are used as a dye and give a soft green colour. An extract is often used to provide silica for supplementation. Horsetail was often used by Indians to polish wooden tools. Equisetum species are often used to analyze gold concentrations in an area due to their ability to take up the metal when it is in a solution.[23]

Extracts and other preparations of E. arvense have served as herbal remedies, with records dating to ancient Greek and Roman medical sources;[20][citation needed] its reported uses include treatments to stop bleeding, treat tuberculosis, to heal wounds and ulcerations, and to treat kidney ailments.[medical citation needed] In modern times, it is typically used as an infusion.[25][26][unreliable medical source?] Reliable modern alternative medicine sources include cautions with regard to its use.[20] In 2009 the European Food Safety Authority issued a report assessing some specific health claims for E. arvense—e.g., for invigoration, weight control, and skin, hair, and bone health—concluding that none could be substantiated.[27]

There is insufficient evidence to draw conclusions regarding its effectiveness as a medicine for all human conditions described.[20] Even so, E. giganteum preparations are widely used in South America as an orally administered diuretic to reduce swelling caused by excess fluid retention and for urinary infections, bladder and kidney disorders.[citation needed] Horsetail preparations contain silicon, so they are sometimes suggested as a treatment for osteoporosis (brittle bone disorders).[28][20]

A female character in the television series Rome (season 2, episodes 7-8) declines an herbalist's offer of horsetail to staunch bleeding after an herb-induced miscarriage (by that character, on a second unsuspecting character).[citation needed]